Printing devices are peripherals for printing characters and/or graphics on papers or other kinds of printing media. Generally, the printing devices are classified into two types: i.e. ordinary printing devices and thermal transfer printing devices. The configurations of the thermal transfer printing devices are substantially identical to those of the ordinary printing devices except for the printing way. For example, the ordinary printing device supplies ink or toner onto a paper. Whereas, the thermal transfer printing device has a thermal print head (TPH) to heat a ribbon and allow the coating of the ribbon to be adsorbed on a transfer paper, so that the image is printed out. The widely-used thermal transfer printing devices include for example faxing machines, POS (Point of Sale) printers and barcode printers.
Recently, a thermal transfer printing device has been introduced into the market. Hereinafter, the structure of a conventional thermal transfer printing device will be illustrated with reference to FIGS. 1 and 2. FIG. 1 is a schematic side view illustrating a conventional thermal transfer printing device. FIG. 2 is a schematic side view illustrating the conventional thermal transfer printing device of FIG. 1 and taken along another viewpoint. As shown in FIGS. 1 and 2, the conventional thermal transfer printing device 1 comprises a casing 10, a thermal transfer printing module 11, a first power device 12, a first gear set 13, a second power device 14, a second gear set 15, a third power device 16, a transmission mechanism 17, and a controlling unit 18. The thermal transfer printing module 11 comprises a ribbon transporting module 111, a transfer paper transporting module 112, a thermal print head 113, and a print roller 114. A ribbon R of the ribbon transporting module 111 is transported through the region between the thermal print head 113 and the print roller 114. A transfer paper M of the transfer paper transporting module 112 is also transported through the region between the thermal print head 113 and the print roller 114. The transfer paper M comprises a releasing paper part M1 and plural medium parts M2. The plural medium parts M2 are disposed over the releasing paper part M1. Every two adjacent medium parts M2 are separated from each other by a gap G. The thermal print head 113 is used for heating the ribbon R to allow the coating of the ribbon R to be adsorbed on the medium parts M2 of the transfer paper M. Consequently, a thermal transfer printing operation is performed to print the medium parts M2 as documents M3. The print roller 114 is used for transporting the ribbon R and the transfer paper M and facilitating the thermal print head 113 to stably perform the thermal transfer printing operation.
As shown in FIG. 1, the ribbon transporting module 111 comprises a ribbon supplying terminal 1111, a ribbon recovering terminal 1112, a first tension shaft 1113, and a second tension shaft 1114. The ribbon R is stored in the ribbon supplying terminal 1111. An end of the ribbon R is wound around and fixed on the ribbon recovering terminal 1112. The first tension shaft 1113 and the second tension shaft 1114 are used for providing tension forces to the ribbon R. As the ribbon recovering terminal 1112 is driven to be rotated, the ribbon R is transmitted from the ribbon supplying terminal 1111 to the ribbon recovering terminal 1112. The transfer paper transporting module 112 comprises a transfer paper supplying terminal 1121, a transfer paper recovering terminal 1122, a third tension shaft 1123, and a fourth tension shaft 1124. The configurations and functions of the transfer paper transporting module 112 are very similar to those of the ribbon transporting module 111. The transfer paper M is stored in the transfer paper supplying terminal 1121. An end of the transfer paper M is wound around and fixed on the transfer paper recovering terminal 1122. The third tension shaft 1123 and the fourth tension shaft 1124 are used for providing tension forces to the transfer paper M. As the transfer paper recovering terminal 1122 is driven to be rotated, the transfer paper M is transmitted from the transfer paper supplying terminal 1121 to the transfer paper recovering terminal 1122.
The relative positions of the components as shown in FIGS. 1 and 2 will be illustrated as follows. The first gear set 13 is connected with the ribbon recovering terminal 1112. The second gear set 15 is connected with the transfer paper recovering terminal 1122. The transmission mechanism 17 is connected with the print roller 114. The first power device 12 is connected with the first gear set 13 for providing motive power to the first gear set 13 in order to drive rotation of the ribbon recovering terminal 1112. The second power device 14 is connected with the second gear set 15 for providing motive power to the second gear set 15 in order to drive rotation of the transfer paper recovering terminal 1122. In addition, the third power device 16 is connected with the transmission mechanism 17 for providing motive power to the transmission mechanism 17 in order to drive rotation of the print roller 114. For example, the first power device 12, the second power device 14 and the third power device 16 are motors. The controlling unit 18 is connected with the first power device 12, the second power device 14 and the third power device 16 for enabling or disabling the first power device 12, the second power device 14 and the third power device 16.
During operations of the thermal transfer printing device 1, the first power device 12, the second power device 14 and the third power device 16 are driven by the controlling unit 18. Consequently, the ribbon R is transmitted from the ribbon supplying terminal 1111 to the ribbon recovering terminal 1112, and the transfer paper M is transmitted from the transfer paper supplying terminal 1121 to the transfer paper recovering terminal 1122. At the same time, the print roller 114 is rotated. When the transfer paper M is transported through the region between the thermal print head 113 and the print roller 114, the thermal print head 113 performs a thermal transfer printing operation. Consequently, the medium part M2 of the transfer paper M is printed as the document M3. As the print roller 114 and the transfer paper recovering terminal 1122 are continuously rotated, the releasing paper part M1 is detached from the document M3. The releasing paper part M1 is transmitted to the transfer paper recovering terminal 1122, but the document M3 is ejected out of the casing 10. Meanwhile, the thermal transfer printing operation is completed.
Before the thermal transfer printing operation is performed on the transfer paper M, a sensing module (not shown) is employed to detect the position of the gaps G of the transfer paper M in order to judge whether the blank medium part M2 is close to the thermal print head 113 and ready to be printed. A conventional sensing module for detecting the gaps G of the transfer paper M is disclosed in U.S. Pat. No. 6,396,070 for example.
FIG. 3 is a schematic perspective view illustrating two sensing modules of a conventional thermal transfer printing device disclosed in U.S. Pat. No. 6,396,070. In FIG. 3, a casing 92, an upper cover 100, a base 102, a first sensing module 10a and a second sensing module 10b of the conventional thermal transfer printing device are shown. The first sensing module 10a is disposed on the upper cover 100. The second sensing module 10b is disposed on the base 102. The upper cover 100 is rotatable relative to the base 102. Consequently, the first sensing module 10a may be rotated to a position over the second sensing module 10b. 
During the process of detecting the gap G of the transfer paper M by the first sensing module 10a and the second sensing module 10b, the first sensing module 10a is served as a light source, and the second sensing module 10b is served as an optical sensing element. Before the gap G of the transfer paper M is detected, the position of the second sensing module 10b should be moved according to the size of the transfer paper M, so that the transfer paper M can be effectively detected. Of course, as the second sensing module 10b is moved, the position of the first sensing module 10a should be correspondingly changed, so that the first sensing module 10a is disposed over the second sensing module 10b. 
The first sensing module 10a is used for emitting a light beam. When the transfer paper M is transmitted to the region between the first sensing module 10a and the second sensing module 10b, the light beam is transmitted through the transfer paper M and received by the second sensing module 10b. Consequently, the gap G of the transfer paper M can be detected by the first sensing module 10a and the second sensing module 10b. 
However, during operations of the first sensing module 10a and the second sensing module 10b, the second sensing module 10b is firstly moved to a detecting position, and then the first sensing module 10a is moved to a region over the detecting position. Consequently, the first sensing module 10a is aligned with the second sensing module 10b. In other words, the first sensing module 10a should be precisely aligned with the second sensing module 10b, so that the light beam from the first sensing module 10a can be received by the second sensing module 10b. Moreover, for detecting the transfer paper M, the task of aligning the first sensing module 10a with the second sensing module 10b should performed as carefully as possible. The design of the conventional thermal transfer printing device is not user-friendly.
Therefore, there is a need of providing an easy-to-use printing device.